High speed, high capacity optical communication systems require high performance devices that introduce minimum degradation. Such devices are required, for example, to introduce minimal insertion losses and should show no spurious reflection peaks or unwanted dispersion slopes. In many applications such as dense wavelength division multiplexing (WDM) transmission systems and satellite communications, optical amplifiers and transmitters, optical filters having ideal amplitude and phase responses are desired in order to maximize bandwidth utilization or to optimise the filtering function. Fibre Bragg gratings are extremely versatile optical components that can provide a large number of filtering functions.
Conventional fibre Bragg gratings are designed using Fourier Transform methods to determine line spacing and strength, and are manufactured using an ultra violet (UV) laser and a phase mask in order to inscribe lines onto an optical fibre. The resulting series of lines creates a filter that in many instances has a frequency response that is not closely matched to the optimal frequency response for the application.
High-speed, high-capacity optical fibre communication systems depend critically on the availability of high performance optical filters to accomplish a number of functions such as selection of closely packed wavelength-division-multiplexed channels or efficient compensation of dispersion over communication links. To this end, there is a strong demand for grating-based optical devices that can be designed and manufactured such that their response is tailored as close to an ideal response as possible. The technology of UV-written fibre gratings has now reached the necessary maturity to implement these high performance filters. There are a number of methods and different approaches in designing high quality grating devices. Among them, Fourier-Transform based and Electromagnetic Inverse Scattering (IS) techniques are known to offer a great variety of possibilities for the design of gratings with various degrees of accuracy.
In order to increase bandwidth efficiency, today's high-speed, high-capacity optical fibre communication systems tend either to decrease the wavelength separation between adjacent wavelength channels and to increase the numbers of channels, or to increase the bandwidth of each wavelength channel. An example of the former is 10 GBit/s data rates on a 25 GHz grid. An example of the latter is 40 GBit/s data rates on a 100 GHz grid. Today's optical filters for inclusion into optical add-drop multiplexers and interleavers for both systems are difficult to design and manufacture. The filters either have insufficient sideband suppression, non-ideal dispersion characteristics, or are insufficiently flat over their bandwidth. It has been recognised that fibre Bragg grating filters that can be designed to meet the required performance are often too long and are consequently difficult to package.
The group delay of a filter is an important parameter that has a significant influence on the distortion of a pulse and on the signal to noise ratio of a communication channel through which pulses travel. The group delay at a wavelength λ0 corresponds to the time taken for optical energy carried by an infinitesimally spaced group of wavelengths around λ0 to be reflected by the filter. It is important that the group delay is approximately constant over the pass band of the filter—that is, that the time taken for each wavelength component in the pulse that is to be reflected (by a grating) is constant. Pulses reflected by such filters which in addition have a substantially constant amplitude response across the filter's passband will not be distorted because each wavelength component of the pulse suffers approximately the same time delay.
Fibre Bragg gratings can be designed to minimize the group delay variation within the bandwidth of the optical filter and to have a near ideal amplitude response. However, 50 GHz or 25 GHz filters designed according to existing theories can be 100 or 150 mm long or even longer depending on the exact spectral shape (squareness etc) of the filters.
Many groups worldwide have looked at this problem and there are many papers on how to design gratings to achieve near ideal performance in both amplitude and group delay performance.
Prior art gratings that are designed and manufactured to have low dispersion, are very long, and more importantly, they are uni-directional. That is, the dispersion characteristic for light input into one end of the grating is optimised at the expense of the dispersion characteristic for light input into the other end of the grating. The uni-directionality is a disadvantage for use in add-drop multiplexers where a single grating is used per added/dropped channel and the dispersion characteristics from both ends are important. However, prior-art gratings are so uni-directional that add-drop multiplexers are designed using one grating to drop a channel and a second grating to add a channel. These gratings are typically separated by an isolator which is a relatively expensive component.
It is an aim of the present invention to provide apparatus for filtering optical radiation at an operating wavelength, which apparatus uses a grating that has the required bi-directional spectral characteristics.